For reading the forum messages, an user doesn't need to be "registered". The [b]non-registered user[/b] is named as the "[b]guest[/b]" of the Forum. Any users or guests can double-click a "forum" title or a "topic" to go inside that message area for browsing the reply messages.
Note that the [b]registered users[/b] only are empowered to create new [b]topics[/b] and post new [b]reply[/b] messages. A registered user is also simply named as the "[b]member[/b]" of the forum.
A "[b]Topic[/b]" is a head thread that other users will follow to post related new messages (called as the new "[b]reply[/b]" messages).